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Outside Micrometer Calibration

Purpose

The purpose of this procedure is to provide general instructions for the calibration of outside micrometers with vernier, digital, or combination scales.

Scope

This general procedure shall be used in the absence of any specific calibration procedure for outside micrometers.

Procedure

Preliminary Instructions and Notes

  1. Read this entire procedure before beginning the calibration.
  2. Calibration shall be performed in an environment that conforms to Manufacturer Specifications.
  3. The outside micrometer will hereafter be referred to as the Instrument Under Test (IUT).
  4. Verify that the IUT is clean.
  5. Visually examine the IUT for any condition that could cause errors in the calibration.
  6. When adjusting micrometer do not retract spindle without carefully stoning the spindle O.D. to remove burrs or nicks (if needed).
  7. Whenever necessary to disassemble for adjustment, use care and cleanliness to assure no damage to the threads.
  8. If any of the requirements cannot be met, refer to the applicable manufacturer manual.
  9. If a malfunction occurs or a defect is observed while calibration is in progress, the calibration shall be discontinued and necessary corrective action taken; if corrective action affects a measurement function previously calibrated, the function shall be recalibrated before the remainder of the procedure implemented.

Specifications

The specifications of the IUT are determined by the applicable manufacturer's documentation. If the manufacturer's documentation is not available, then the specifications identified in this procedure are used.

Micrometers with 0.0001" graduations shall be within 0.0001".

Micrometers with 0.001" graduations shall be within 0.001".

Reference Material

Applicable Manufacturers Manual or Brochures.

Equipment Required

The Standards listed below should be selected on the basis of their higher accuracy level when compared to the unit under test. Equivalent Standards must be equal to or better than the Minimum-Use-Specification.

Minimum-Use-Specifications for Standards listed are 1/4 the accuracy required by the IUT.

  • Cleaning solution
  • Hard arkansas stone
  • Lint free cloth
  • Gage oil
  • Gage block set

Detailed Procedures

  1. Clean faces and all exterior surfaces with lint free cloth, dampened with cleaning solution.
  2. Close IUT using ratchet or friction stop.
  3. Hold IUT to light source and visually examine for light showing between faces. If faces are not parallel, light will show between them.
  4. Remove spindle assembly.
  5. Clean then oil spindle and measuring screw.
  6. Clean and oil fixed nut in barrel.
  7. Reassemble IUT.
  8. Check measuring screw for wear by pushing thimble to and fro in direction of measuring screw axis. There should be no movement.
  9. Adjust for wear by tightening fixed nut on barrel to a smooth, tight, (no shake) fit for the full thread length. A smooth tight fit must be achieved for acceptance.
  10. Close IUT using ratchet or friction stop to check zero setting.
  11. Check accuracy with gage block(s) having accuracy not less than 0.00001 inch. Block sizes must test the IUT not only at complete turns of the thimble scale, but also at intermediate positions. This is required as a check on the accuracy of the scale around the thimble as well as the measuring screw.
  12. Place block or block combinations between anvil and spindle of the IUT and close the IUT using ratchet or friction stop. Use block combinations to check the accuracy of the instrument in three positions of total thimble range (zero, midway & full length).

Reason for Reissue

Revision A - First Release

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